The present invention relates to gas turbine engines for aerospace propulsion and more particularly to a fluid cooling arrangement for use with such gas turbine engines, as well as a method of cooling a fluid.
Modern aircraft gas turbine engines operate at ever-increasing turbine temperatures and pressure ratios in order to optimize the open Brayton cycle on which they are based. Advances in materials, turbine cooling techniques and electronic fuel controls, for example, have allowed these efforts to progress substantially through the years.
Typically, each turbine in a multi-spool axial-flow gas turbine engine is mounted within a turbine case. The rotationally mounted rotor blades in each turbine extend radially from its associated spool and rotate due to the expanding hot gases emanating from the upstream combustor. The turbine is rigidly connected to the compressor via a shaft that allows the compressor to rotate and ingest ambient air. Typically, on twin-spool engines, a low pressure turbine (LPT) and low pressure compressor (LPC) are connected by the same shaft. Concentrically surrounding it, and mechanically independent is the shaft which connects the high pressure turbine (HPT) and the high pressure compressor (HPC). A large engine fan is also connected rigidly to the LPC or may be coupled to the LPC via a gear reduction mechanism. The fan is surrounded by a fan case connected to the engine case via support struts. Three-spool engines simply add a third concentric shaft between the low-pressure spool and the high-pressure spool, connecting the intermediate pressure turbine (IPT) to the intermediate pressure compressor (IPC). The various shafts are supported by a series of bearings (ball and/or roller).
During a typical gas turbine engine operating cycle, bearing lubricating oil may reach very high temperatures thus limiting its cooling/lubricating capabilities. In extreme hot conditions (e.g., ground hot day at idle at a high-altitude airport and take-off), oil coking may cause oil cooling passages blockage which in turn may lead to oil starvation resulting in damaged main engine shaft bearings. Therefore, it is important that main engine oil is maintained within its optimal operational temperature range. This ensures that its viscosity, lubricity and pour points remain near their optimal design values. Hence, active cooling of the engine bearing oil is necessary to maintain these operational requirements.
Many attempts have been made to overcome the problem of overheated engine oil by designing various air-to-oil coolers using compressed fan air as the cooling sink. Despite their partial success in mitigating these problems, typical installation of the oil tanks and the related heat exchangers (oil coolers) are located around the engine case and prone to damage during a rotor burst event. In addition to being vulnerable to damage, the associated oil cooling hardware adds weight, complicates maintenance, and operational cost. Hence, there is a need for advanced gas turbine engine oil cooling concepts that address one or more of the above-noted drawbacks.